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  #1  
Old 07-03-2010, 12:30 PM
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Join Date: May 2009
Location: Northern Calif.
Posts: 6
300SD Vacuum Problems

I find myself working on an '82 300SD for a friend of my mother's. I've done a lot of Internet research and a lot of tests based on that research, but still have no definite answers and decided to post for some opinions. Here's what's going on, and what I've done so far:

Vacuum problems seem typical for this model, I.E. power brakes are intermittent. Drive it a bit and you get one good braking action, but step on the brakes a second time within 30 seconds or so and there's not enough vacuum to actuate the booster so the pedal is hard and ineffective. Engine does shut off with the key, though it reacts slowly at times (when the vacuum is low, of course).

I've tested the system with a gauge connected to one of the Tees in the main line from the pump to the brake booster, it does pull a vacuum. But it's not the greatest...start the engine and run it at an idle and the vacuum rises slowly, topping out at about 18-20". Disconnect and pug everything except the brake booster to eliminate any other source of vacuum loss and it tops out at 22" but still takes about 40 seconds at idle to get there, and drops like a stone as soon as the engine is shut off.

I've checked for leaks with my mitey-vac, and found a few...but let's discount those for now and concentrate on just the vacuum pump and brake booster as I suspect that one of those is the main source of the problem since it still exists even with all other vacuum ancillaries disconnected and the lines plugged, running only the main line from the pump to the booster. I even took it for a (short) drive that way, braking is still as before.

I've hooked my vacuum gauge directly to the pump and it rises to almost 30" within a few seconds...but still drops to zero just as quickly when the engine it shut off. It's supposed to hold vacuum, isn't it? At least for a while...?

I've pulled the pump and disassembled, inspected and cleaned it. The check valves all look great, the piston, ring/band, o-ring, actuator arm, and even the bearing all look good and function smoothly. The piston with ring/band/whatever you call the thing, seems tight in the bore.

I've tested the external screw-in check valve by taking off the hard line that attaches to it and hooking my vacuum gauge and mitey-vac
to the other end and pumped it down. It holds vacuum for the most part, but does bleed back down to zero within about 10 seconds. I've cleaned and cleaned it but can't get any better results than that. Looking down inside it, it appears to be in good shape.

I've also checked the line between the pump and the booster...no leaks there, it holds vacuum just fine.

As for the booster...I've hooked my mitey-vac directly to it and pumped...and pumped...and pumped. I can get it down to about 12-15" with a hell of a lot of pumping (that's a large volume of air in there!) and it holds a vacuum with no problem...but I'm not sure if that's the most accurate or appropriate way to test it.

The owner can't afford to just blindly throw parts at it (and that's not the way I like to work anyway) so I'd really like to get some idea of which direction to go at this point without just sticking another booster or vacuum pump on it. Any help would be appreciated.

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  #2  
Old 07-04-2010, 09:53 AM
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Join Date: May 2002
Location: Blue Point, NY
Posts: 25,396
Good job with troubleshooting.

You've effectively ruled out the vacuum pump. The performance is excellent.

However, the check valve should be replaced...........leaking down in 10 seconds isn't desirable, although it's not likely the source of your vacuum problems.

The booster appears to be OK...........doesn't seem to leak down..........which is the definitive test for it.

Therefore, the only culprit is a vacuum leak external to the booster. You've disconnected the remainder of the system and the pump/booster combo works as designed. Therefore, a significant leak in one of the other consumers is the issue. This must be isolated with the Mity-vac.

One culprit that can be completely eliminated on that vehicle is the EGR system. The two valves inside the black box are noted for leaks and they only serve to run the EGR system. The supply vacuum to the black box can be removed and plugged at the T. In fact, the entire T can be removed with a short connection hose piece inserted in place of it. This can also be done on the T for the vent (the second line to the black box).

If the EGR system is not the culprit, the climate control system can certainly cause such issues if any of the pods are seriously leaking.

Most of the consumers have an orifice in the supply line, of some type, to prevent the anomaly that you are experiencing. The orifice serves to isolate the booster and vacuum pump from a seriously leaking consumer, thereby avoiding the problem of insufficient brake action.

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